Typically, ammonium phosphate fertilizers are produced from wet process phosphoric acid. For several years the quality of phosphate rock being mined in Florida has been declining. This is important because wet process phosphoric acid is produced by decomposing natural phosphate rock with sulfuric acid and filtering off the precipitated calcium sulfate. Depending upon the quality of the phosphate rock processed, the resulting crude phosphoric acid solution contains varying amounts of impurities such as iron, aluminum, magnesium, calcium and other metals in the form of their phosphates. The impurities act as diluents during the manufacture of ammoniated phosphate fertilizers and often prevent obtaining the required grade specifications pertaining to nitrogen and available phosphate content. Consequently, the wet process phosphoric acid manufactured from the phosphate rocks has become increasingly impure. As a result, it has become increasingly more difficult to produce and maintain industry acceptable grades of ammonium phosphate fertilizers.
The impurities in the wet process phosphoric acid which cause the greatest problems are primarily iron, aluminum and magnesium phosphate which replace hydrogen ions, thereby leaving fewer hydrogen ions available for ammoniation. Additionally, some of the impurities, primarily iron, form upon ammoniation insoluble phosphate compounds thereby reducing the available P.sub.2 O.sub.5 content of the ammonium phosphate fertilizer.
Tests of the remaining phosphate rock deposits in central Florida indicate that the quality of the rock will decline even further. This will make it even more difficult, if not impossible, to produce the currently acceptable grades of ammonium phosphate fertilizers.
Thus, it is the object of the present invention to provide a process for removing at least a portion of the impurities in wet process phosphoric acid, and thus maintaining the grade of granulated mono- or di-ammonium phosphates at the current acceptable industry standards.
Another object of our invention is to provide a processing step that can be integrated into existing ammonium phosphate plants and permits the partial and controlled removal of the above-mentioned impurities. For economic reasons it is benefical to remove only as much of the impurities as is necessary to insure the grade specifications of the desired di-ammonium or non-ammonium phosphate products.